The present invention relates to apparatus and methods for proving the accuracy of gas flow meters and, more particularly, to such apparatus and methods particularly adapted for in-situ testing of large capacity gas meters of the rotary positive displacement type.
The gas flow measuring accuracy of gas meters utilized by natural gas utility companies to monitor and determine billing for the quantity of gas usage by its customers is, of course, a matter of significant concern and importance, particularly with respect to gas meters of relatively large capacities, e.g., in excess of 30,000 CFH (cubic feet per hour), such as utilized by manufacturing companies and other industrial customers. In most states, the governing body overseeing the operations of public utility companies specify that the accuracy of metering equipment shall be tested by comparison of the meter's accuracy with the known accuracy of a standard, such as that provided by so-called transfer testing wherein a volume of air is passed through both a meter to be tested and a calibrated reference meter to permit comparison of the respective gas flow measurements generated by each meter. Portable so-called meter proving equipment is commercially available for this purpose but characteristically is capable of handling only relatively low rates of air flow in the range of less than 12,000 CFH. Accordingly, inasmuch as it is necessary that a meter be tested in the range of 50% to 80% of its full capacity to obtain an acceptable test of the meter's accuracy, such proving equipment is wholly incapable of providing reliable testing of large capacity gas meters and instead is utilized only for proof testing of residential and other small capacity gas meters.
Accordingly, whenever it becomes necessary to test the accuracy of large capacity meters in field use, such as when requested by the customer in response to an apparently unusually high meter reading, it has been common industry practice to conduct differential testing in the case of meters of the rotary positive displacement type and spin testing of meters of the turbine wheel type. Specifically, in differential testing of rotary type meters, manometer testing is conducted at the inlet and outlet of the meter to determine the differential in the gas flow rate on each side of the meter. In the spin testing of turbine type meters, the turbine wheel of the meter is spun to a given speed and the time required for the wheel to slow to a stop is measured. Each type of test essentially only determines whether or not any unusual source of friction exists within the mechanical working components of the particular test meter, which only indirectly provides an indication as to the flow measuring accuracy of the meter. Other than this type of testing, the only remaining feasible manner of proving the accuracy of a large capacity meter is to take it out of service and return it to the manufacture for proof testing, which of course is ordinarily prohibitively expensive.
One of the practical problems in designing an apparatus for proof testing of large capacity gas meters is that a predominant number of such meters in service are of the rotary positive displacement type wherein a rotary valve member is adapted during rotation to pulsingly capture and convey essentially discrete measured quantities of gas which, when connected in line with a reference meter, greatly disturbs the measuring accuracy of the reference meter. Specifically, the pulsations inherently created by such a rotary type meter typically cause the reference meter to measure more than the actual gas volume passing therethrough and, in comparison, the rotary test meter therefore appears to be measuring less than actual gas volume, thus producing an invalidly low percentage proof calculation for the rotary test meter. This problem perhaps as much as any has retarded efforts to develop a reliable meter proving apparatus for mobile use in field testing large capacity rotary type gas meters, whereby only very large, non-mobile, and expensive meter proving equipment such as that typically maintained in the manufacturing facilities of meter manufactures is known to exist for accurately proving the flow measuring accuracy of such meters.
In contrast, the present invention provides an inexpensive reliable proving apparatus capable of being conveniently mounted on an ordinary flat bed truck or housed in an ordinary van-type truck for on-site transfer testing of large capacity rotary gas meters in field use.